This invention relates to processing of materials and more particularly to apparatus for processing materials in space under ultra-high vacuum levels.
Attainment of the ultra-high vacuum conditions required for certain material processing applications has been a continuous problem. For example, in growing thin films by molecular beam epitaxy (MBE), atoms are typically being deposited on a target surface at the rate of 10.sup.15 cm.sup.2 /sec, which corresponds to a film growth of 1 micron per hour. At this growth rate a background vacuum level of 10.sup.-11 to 10.sup.-12 torr can incorporate as much as 1 ppm impurity if the contaminating molecules chemisorb on the surface. This would present an unacceptably high dopant level if the impurity is electrically active. Residual backgrounds of H.sub.2 O, O.sub.2,CO, CO.sub.2, CH.sub.4, N.sub.2, various silicones and metal or semi-metal atoms are particularly troublesome. The MBE process is especially susceptible to chamber contamination, and great care must be taken to avoid introducing any extraneous materials into an MBE chamber for fear of ruining a very expensive facility. This essentially requires a dedicated chamber for each particular type of process and restricts experimentation with various materials, for example mixed III-V/II-VI systems. As a result of these difficulties MBE has been kept in the realm of a research process and its development has been retarded despite is significant promise for growing precision epi-layers and superlattices.
Conventional ultra-high vacuum chambers can attain vacuums in the rage of 10.sup.-10 to 10.sup.-12, but they require extensive bake-out and other time-consuming maintenance. In addition to the problems mentioned above, these systems have difficulty in maintaining good vacuum in the presence of large gas loads such as those involved in growing thin films by metal-organic chemical vapor deposition (MOCVD) or in the presence of high heat loads in deposition on substrates that are heated to improve the mobility of the ad-atoms, or in the ultra-purification processes where samples are heated to remove impurities by evaporation or other means.
Use of the vacuum environment of outer space for performing ultra-high vacuum processing has been proposed for many years. In general the prior concepts use a shell-shaped wake shield in which the process is carried out, the wake shield being disposed to trail behind an orbiting vehicle. In the concept studied in most detail a hemispherical shell wake shield would be attached by a tether or long telescoping boom and flown behind the Space Shuttle with the open end of the hemisphere facing the wake direction. The material being processed as well as process equipment and support packages would all be located on the concave side, facing the wake direction. This concept further envisioned closing the shield with a hinged matching hemisphere, pumping the system down and baking it out prior to loading on the Shuttle. Auxillary pumping would likely be required to maintain the vacuum until deployment on orbit, when the hemisphere would be opened.
The prior wake shield concept presents weaknesses in obtaining the desired ultra-high vacuum levels. The walls of the hemisphere, which partially surround the surface being processed, can act as a source of contamination and negate some of the advantages of using the vacuum of space. Also, the curved portion of the walls face in the ram direction. Residual atmospheric molecules striking these walls can become accommodated and re-emitted with thermal velocities. The geometry of the system is such that these thermal molecules can get behind the shield. Since they are traveling at much slower velocities than the ambient molecules, the thermal molecules can backscatter ambient molecules into the region protected by the shield. This would probably be the limiting factor in the level of vacuum attainable by such a shield.
It is therefore an object of the invention to provide an ultra-high vacuum processing facility in which contamination is virtually eliminated.
Another object is to provide a wake shield space vacuum facility in which backscatter of contaminants onto surfaces being processed is minimized.
Another object is to provide a method of deploying a wake shield facility in a manner such as to provide for cleaning and bake-out of the shield surface on the side where processing is carried out.